In the past calcium fluoride has been produced by various dry processes. For instance, U.S. Pat. No. 2,573,704, N. Gilbert et al., teaches making calcium fluoride by reacting calcium carbonate with gaseous hydrogen fluoride. In this process hot waste gases containing hydrogen fluoride are passed through a bed of 1/4-2 inch oolitic and pisolitic limestone. The hot gases react with the outer surface of the limestone, producing calcium fluoride which decrepitates as a powder. The calcium fluoride rich portion is separated by screening, and the coarser material composed mainly of limestone is recycled.
Another dry process is described in Ross's U.S. Pat. No. 3,357,788 wherein calcium carbonate is reacted with ammonium bifluoride or with ammonium fluoride. The calcium fluoride produced has an average particle size of about 8 microns.
Both of these dry process patents refer to wet processes as the undesirable approach to produce calcium fluoride. The Ross patent states that large volumes of solutions must be held at their boiling temperature for several hours in order to develop calcium fluoride crystals from extremely fine particles to desirable sizes such as about one micron average diameter.
The Gilbert patent states that various types of wet treatments of partially reacted limestone for removal of calcium fluoride have been suggested, but that these methods are generally too expensive and complicated for commercial production.
Other patents of interest include U.S. Pat. Nos. 3,551,332; 3,529,929, and less pertinent 2,213,907, 2,781,244, 2,781,245, and 2,839,369.
The search conducted was in Classes 23, 423 and 210.
The processes in the known art produce a finely-divided powder or small particles in a large volume of liquid which requires filtration and drying. The drying step is necessarily difficult, and energy and time-consuming because the calcium fluoride is finely divided and thus difficult to dry.
With the present emphasis on energy and mineral conservation throughout the world, there is real value in the improvements provided by the present invention. Not only does this invention offer a method of removing fluorides from solutions that might possibly cause disposal problems, but also produces a potentially valuable product that can easily be dried with essentially no dust or handling problems.